An actuating solenoid of the generic type is employed, for example, as a solenoid actuator or proportional solenoid for directional control valves in order to adjust their valve spools. The basic structure of such an actuating solenoid is explained, for example, in WO 99/62084. According to this publication, an actuating solenoid has a pole tube that accommodates an armature which can be moved axially and by means of which the valve spool can be adjusted directly or indirectly. The pole tube is surrounded by a coil so that, when the coil is energized, the armature can be moved out of its spring-biased initial position.
The function and the structure of a pole tube are known from the book titled “Die Bibliothek der Technik” [The library of technology], volume 118; Linsmeier, Klaus-Dieter; published by Verlag Moderne Industrie, 1995. According to this publication, a pole tube of an actuating solenoid has a pole core on its face and is joined to a tube piece or yoke by means of a spacing ring made of non-magnetic material. An armature is accommodated movably inside the pole tube, the end stroke position (operating stroke) of the armature being determined by the contact to a face of the pole core or to an inserted non-stick disk. In the transition area between the spacing ring and the pole core, a so-called control cone is formed that widens in the direction of the stroke. The geometry of this control cone is selected in such a way that the characteristic curve of the solenoid runs essentially linearly. Normally, the interior of the pole tube is charged with a high pressure that corresponds, for instance, to that of the associated valve-controlled system pressure.
When the coil is de-energized, the armature is moved by the spring bias out of its contact position on the pole core in the direction of its initial position. The non-stick disk arranged between the pole core and the face of the armature is intended to prevent the armature from adhering to the pole core. During this backward movement of the armature, the oil contained in the pole tube has to flow out of the decreasing part of the armature space and into the increasing part of the armature space that is delimited by the above-mentioned face of the armature. The disable time, that is to say, the time that the armature needs to move out of its stroke position in the direction of its initial position, should be as short as possible, so that the valve exhibits adequate response characteristics. One prerequisite for a short disable time is that the above-mentioned oil equalization between the armature spaces has to take place as quickly as possible. In actual practice, it has proven that this oil equalization cannot be performed at the requisite speed by means of the above-mentioned solutions.